Polyurethane foam is prepared commercially in the form of large blocks that are subsequently cut into the desired shape for use in the manufacture of various articles that require a foam padding. It is well known that polyurethane foam requires time to develop its full physical properties. In typical polyurethane foam formulations, a polyol, water and diisocyanate are reacted in the presence of catalysts or other additives. Much of the time a small percentage of terminal isocyanate groups are left unreacted in the foam structure. If the foam is distorted or compressed in this condition, it fails to recover its original dimensions when it is distorted or compressive force is released. Normally, the terminal isocyanate groups that are left unreacted in the foam structure will react with the residual water in the foam structure or with the water vapor in the atmosphere over a period of several hours or days, and the foam will achieve its full physical properties.
As pointed out in the Encyclopedia of Polymer Science and Technology (John Wiley and Sons, New York 1969) in the section on polyurethanes, polyethers are commercially the most important of the polyhydroxy compounds ("polyols") used to prepare polyurethanes. At the present time most of the polyethers used in the production of flexible polyurethane foams are derived from propylene oxide and ethylene oxide. In this preparation propylene oxide is reacted with glycerol in the presence of a basic catalyst to form a poly(oxypropylene) homopolymer which is further reacted with ethylene oxide to form the block copolymer.
Poly(oxytetramethylene) glycols are prepared by the polymerization of tetrahydrofuran. Poly(oxypropylene) triols are at present the most important class of polyethers used in the manufacture of polyurethanes. These triols are prepared by the same general reactions as poly(oxypropylene) glycols.
The most important common monomers used in polyesters for the preparation of urethane polymers are adipic acid, phthalic anhydride, ethylene glycol, propylene glycol, 1, 3-butylene glycol, 1, 3-butylene glycol and diethylene glycol. The polyurethanes derived from polyesters do not normally present post-curing problems. The term "polyether polyurethane" as used throughout this application refers to polyurethanes derived from polyether polyols.
The process of the instant invention provides a rapid method to achieve rapid and full post cure of polyether polyurethanefoam so that the foam will achieve low compressive set values as measured by the standard compression set test (Constant Deflection Compression Set Test, ASTM D-3574). The process reduces the total post curing time from a matter of many hours to less than 5 minutes. Using this process, most types of polyether polyurethane foam can be cured, fabricated and shipped in less than 24 hours.
U.S. Pat. No. 3,061,885 to Rogers and Peabody discloses a process in which pressurized air in blasts is applied to penetrate the foam block. The process covered in this patent opens up the cell structure of the foam and makes it more porous.
U.S. Pat. No. 3,890,414 to Ricciardi et al treats a freshly prepared foam, i.e. 15 to 240 minutes after it leaves the foam-forming area with air or other non-reacting gas. The inventors state by cooling the foam the physical properties are made more uniform in a shorter period of time than is required to normally develop these properties.
U.S. Pat. No. 3,723,393 to Kistner discloses a process in which isocyanate or haloformyl terminated hydrophylic polyoxyalkylene prepolymers are reacted with certain compounds containing releasable hydrogen atoms such as hydroxy, thiol, amino, amide, ammonia, or primary amines, to form a hydrophilic amide-endcapped prepolymer. The prepolymer is cross-linked with a cross-linking agent under acidic conditions to provide a cured hydrophilic material.
The post curing of polyurethane foams is dicussed in "Plastic Foams", Part I, by Frisch and Saunders (M. Dekker Inc., New York 1973) at pages 232 to 235.